Amplifier Output Impedance (Damping Factor) and Speakers

[KSTR]

The damping of a speaker driver is determined by the sum of all resistances in the path, which comprises of the voice-coil resistance (dominant factor), any XO components in series, and the amp output impedance (negligible).
Thus, for the damping it does not matter if the woofer sees its own DC resistance, say, 3.5 Ohms or a little bit more, like 3.7Ohms (200 Milliohms of XO, wiring and output impedance), a change of only 5%. Voice coil resistance changes from heating up the VC under load simply drowns any reasonable DF anyway.
Importance of DF is a concept that was invented by marketing guys ;-)

 

[pma]

In a classical multi-way passive speaker box with R, L, C elements crossover filter you do not want the box input voltage frequency response to be modulated as a result of Zout/(Zout + Zspk) divider function. Thus reasonable DF (at least 50 IMO) in the whole audio range makes sense. You do not want Zout of 1 ohm.

 

[KSTR]

@pma,
yes, high DF helps getting the proper intended response without going into the depths of correct EQ'ing which is not feasible for the lay person.
But the DF 1000 vs DF 30 dicussions is useless.

 

[Suffolkhifinut]

As long as you maintain the exact same frequency response at the speaker terminals by proper pre-EQ, no difference when going from DF 1000 to DF 10, and almost no change when going to DF 1. If not EQ'd, you'll hear the frequency response change of course (unless the speaker is fully impedance compensated).
Only when you approach current drive (DF 0) it can sound a little different even when exactly EQ's to the same target. Reason explained here.

Looked at your reference post ‘here’ and have to agree with the subsequent reply post ‘1,230’, given by egellings. Again back to Lenz’s Law, if you supply current to an inductive load the back EMF will oppose the rise in circuit current. If the supply current is reduced or removed the voice coils will have an induced EMF that will oppose the decrease and will act as a supply source. As egellings said lower amplifier impedance will have quite an effect on the self induced speaker movement. The difference will be more noticeable in some speakers than others.

 

[Geert]

As egellings said lower amplifier impedance will have quite an effect on the self induced speaker movement.

Not if the amount of amplifier output impedance is negligible compared to other resistance in the path. Or would it make a difference for bas reflex speakers with extreme low impedance at the box resonance frequency? If the box is designed with an appropriate Qtc I don't expect any impact.

 

[Suffolkhifinut]

Not if the amount of amplifier output impedance is negligible compared to other resistance in the path. Or would it make a difference for bas reflex speakers with extreme low impedance at the box resonance frequency? If the box is designed with an appropriate Qtc I don't expect any impact.

With an infinite baffle design damping is less critical, as the sealed box will provide some mechanical damping. The freer the speaker is to move such as in ported designs the more critical damping eill become.

 

[Geert]

With an infinite baffle design damping is less critical, as the sealed box will provide some mechanical damping. The freer the speaker is to move such as in ported designs the more critical damping eill become.

Well yes, that's pure speaker basics. But how to quantify "the more critical damping will become". If a ported enclosure is designed with a low Qtc it's already damped sufficiently. And then there's still the total system impedance to be compared to a very low amplifier output impedance.

 

[Jim Shaw]

Good poop. However, the comments remind me of the blind men and elephant allegory.

You have me straining mightily to remember my coursework on driving point impedances, poles and zeros, and acoustic drive coupling impedances. But all of that matters -- especially conceptually -- in understanding what's going on. Quantizing it for a generalized system is elusive. And, the concept of negative driving point impedances, which can work near miracles in acceptable signal transfers in physically realizable systems. (Consider the advantages of a negative number as a damping factor.)
...
Generalizing down to "that's why solid-state amplifiers sound different than tube amps" does a disservice to systems understanding. That's when mathematical turns into political, and things get tribal.

 

[Suffolkhifinut]

Well yes, that's pure speaker basics. But how to quantify "the more critical damping will become". If a ported enclosure is designed with a low Qtc it's already damped sufficiently. And then there's still the total system impedance to be compared to a very low amplifier output impedance.

A very low amplifier output impedance will virtually short circuit the speakers parallel load impedance. If the amplifiers output impedance is extremely low the speakers impedance doesn’t matter.
On the differences in the ‘Tube v Solid State’ debate, often overlooked is the way biasing affects their operation.

 

[DSJR]

I apologise for jumping into this thread when I have no objective or academic reasoning but can you bear with me a little here?

The amps which started (for me) the entire 'damping factor' thing were the 70's Crown models, which came with superb user/tech manuals (downloadable from the Crown site) and much play was made about it, even a chart drawn to show what happens with different gauges of speaker cable and distance from amp to speaker. Back then, we sold a good few IMF transmission line speakers with KEF B139 bass units and in the pre-1976 models, if you gently pushed in the bass unit's diaphragm and let go, it visibly 'wobbled' back and forth a few times before coming to rest. The 'BBC LS3-6/Spendor BC1/KEF 104' family of speakers also had a pretty under-damped 'loose' bass driver loading (BC1 described on the Gravesen site) and these used to bottom out all too easily with vinyl sources (high compliance pickups in too-high mass tonearms) with no sub filtering in the phono stages. Compared to many other amps, said Crowns did seem to offer a tauter bass 'quality' for whatever reason to such 'wobbly bass' speakers (they measured better back then as well, still good enough today to be more than quite acceptable if the Ken Rockwell tests can be believed)

The only practical thing I can add (again, purely subjective) is comparing my D-60 stereo model (accepting it's only 40WPC typical output) and two sets of bridged pairs of D-60's I inherited. Into 'BeeBeeCee' loose-bass boxes, the bridged set do seem to magnify the speaker's existing 'bass/top character' a little over the single stereo D-60 (which is similar to me, to a much loved D-150 I use regularly).

P.S. You lot must remember that where the better UK speakers back then could have superb midrange, they often went to pieces in the bass and the US imports I heard usually were in a different world in the low bass region regardless of amp used. Being in my late teens in the mid 70's the larger JBL's and AR models really did appeal (I cite 'Halleluwah!' by Can played quite loudly [cough] as a reference track from the times... BC1's used to sound as if they were shaking apart at not high volumes and they probably were ). Sorry for the subjective memories and anecdotes, but that kind-of explains my continuing interest in this topic...



I couldn't help it, here's a heavily edited sample of the Can track - Lord it sounds distorted by today's standards..

 

[Suffolkhifinut]

I apologise for jumping into this thread when I have no objective or academic reasoning but can you bear with me a little here?

The amps which started (for me) the entire 'damping factor' thing were the 70's Crown models, which came with superb user/tech manuals (downloadable from the Crown site) and much play was made about it, even a chart drawn to show what happens with different gauges of speaker cable and distance from amp to speaker. Back then, we sold a good few IMF transmission line speakers with KEF B139 bass units and in the pre-1976 models, if you gently pushed in the bass unit's diaphragm and let go, it visibly 'wobbled' back and forth a few times before coming to rest. The 'BBC LS3-6/Spendor BC1/KEF 104' family of speakers also had a pretty under-damped 'loose' bass driver loading (BC1 described on the Gravesen site) and these used to bottom out all too easily with vinyl sources (high compliance pickups in too-high mass tonearms) with no sub filtering in the phono stages. Compared to many other amps, said Crowns did seem to offer a tauter bass 'quality' for whatever reason to such 'wobbly bass' speakers (they measured better back then as well, still good enough today to be more than quite acceptable if the Ken Rockwell tests can be believed)

The only practical thing I can add (again, purely subjective) is comparing my D-60 stereo model (accepting it's only 40WPC typical output) and two sets of bridged pairs of D-60's I inherited. Into 'BeeBeeCee' loose-bass boxes, the bridged set do seem to magnify the speaker's existing 'bass/top character' a little over the single stereo D-60 (which is similar to me, to a much loved D-150 I use regularly).

P.S. You lot must remember that where the better UK speakers back then could have superb midrange, they often went to pieces in the bass and the US imports I heard usually were in a different world in the low bass region regardless of amp used. Being in my late teens in the mid 70's the larger JBL's and AR models really did appeal (I cite 'Halleluwah!' by Can played quite loudly [cough] as a reference track from the times... BC1's used to sound as if they were shaking apart at not high volumes and they probably were ). Sorry for the subjective memories and anecdotes, but that kind-of explains my continuing interest in this topic...



I couldn't help it, here's a heavily edited sample of the Can track - Lord it sounds distorted by today's standards..

In the 1970s had an AR amp and you could crack walnuts with the bass and it was relatively low powered. Still working having given it to a neighbour.

 

[pogo]

The damping of a speaker driver is determined by the sum of all resistances in the path, which comprises of the voice-coil resistance (dominant factor), any XO components in series, and the amp output impedance (negligible).

If I understand this correctly, see here: Link, the dominant factor can be very variable in reality, especially with transients, which in turn can have different effects depending on the design.

 

[Suffolkhifinut]

If I understand this correctly, see here: Link, the dominant factor can be very variable in reality, especially with transients, which in turn can have different effects depending on the design.

Hegel amps have a very high damping factor and generally get very good reviews. Is it a coincidence?

 

[DonH56]

For the record, I started this mess, but have not kept up (Life and Work, busy) nor read every post.

Voice coil impedance is generally not considered in these discussions but rather the driving point impedance, what the amp sees looking into the speaker and the speaker sees looking out the terminals heading back to the amplifier. Amplifier output impedance and wiring (speaker cables) plus contact resistances usually dominate. Looking into the speaker, the usual concern is the crossover, its impedance and what driving (source) impedance it was designed to accommodate. Frequency variations IME are often a function of the impedance the crossover sees, which again is the amp's output impedance (1/DF) and wires to the speaker. Of course driver impedances are translated back through the crossover to the amp, and ported speakers have impedance nulls at port resonance, etc. -- it's a system.

Frequency and time domain responses are related so anything seen in one domain can be seen in the other. You may need the right test signal(s) and note frequency response has magnitude and phase, just as time response has magnitude and duration.

Someone asked me about frequency response variations, were they audible, how large, etc. I refer to the first couple of posts wherein graphs of responses are shown for some example amps and speakers; sometime, you have to go back to the beginning, and not get lost in the tens or hundreds of posts after.

FWIWFM - Don

 

[Suffolkhifinut]

For the record, I started this mess, but have not kept up (Life and Work, busy) nor read every post.

Voice coil impedance is generally not considered in these discussions but rather the driving point impedance, what the amp sees looking into the speaker and the speaker sees looking out the terminals heading back to the amplifier. Amplifier output impedance and wiring (speaker cables) plus contact resistances usually dominate. Looking into the speaker, the usual concern is the crossover, its impedance and what driving (source) impedance it was designed to accommodate. Frequency variations IME are often a function of the impedance the crossover sees, which again is the amp's output impedance (1/DF) and wires to the speaker. Of course driver impedances are translated back through the crossover to the amp, and ported speakers have impedance nulls at port resonance, etc. -- it's a system.

Frequency and time domain responses are related so anything seen in one domain can be seen in the other. You may need the right test signal(s) and note frequency response has magnitude and phase, just as time response has magnitude and duration.

Someone asked me about frequency response variations, were they audible, how large, etc. I refer to the first couple of posts wherein graphs of responses are shown for some example amps and speakers; sometime, you have to go back to the beginning, and not get lost in the tens or hundreds of posts after.

FWIWFM - Don

Don,
Think voice coil impedance has been mentioned. Audio reproduction has many factors and variations for all the graphs and measured data. Our ears are the ultimate measuring system.

 

[DonH56]

Don,
Think voice coil impedance has been mentioned. Audio reproduction has many factors and variations for all the graphs and measured data. Our ears are the ultimate measuring system.

Yes, saw that, why I piped in. As for the rest, I do have some limited insight into audio production and reproduction and all that jazz, been piddling with it for a while. But as a measurement system, I don't think ears are all that great, as many instruments have far more dynamic range and resolution. However, for us as individuals, in the end what matters is what we hear. (Or think we hear... )

 

[DSJR]

In the 1970s had an AR amp and you could crack walnuts with the bass and it was relatively low powered. Still working having given it to a neighbour.

I seem to remember the original AR amp had transformer coupled speaker outputs? I have to say I loved it back then...

 

[KSTR]

If I understand this correctly, see here: Link, the dominant factor can be very variable in reality, especially with transients, which in turn can have different effects depending on the design.

Indeed output impedance of an amp (and its rescaling to "damping factor") can vary a bit with dynamic operating conditions (which directly translates to additional distortion).
Still output impedance does never dominate the damping the drivers sees as long as it remains low in comparision to the driver impedance. Below 1/10th (DF > 10) it has zero effect on damping...

 

[pogo]

Indeed output impedance of an amp (and its rescaling to "damping factor") can vary a bit with dynamic operating conditions (which directly translates to additional distortion).

Can you deposit 'vary a bit' with numbers/facts? This goes exactly in the direction where I never received any feedback from Benchmark, see also here: Link

 

[SIY]

Indeed output impedance of an amp (and its rescaling to "damping factor") can vary a bit with dynamic operating conditions (which directly translates to additional distortion).
Still output impedance does never dominate the damping the drivers sees as long as it remains low in comparision to the driver impedance. Below 1/10th (DF > 10) it has zero effect on damping...

^^^^
And yet, that simple calculation seems to be beyond @pogo to bother to perform.